The invention herein relates to transition logic circuits for use between logic gate circuits from different logic families and, more particularly, to transition circuits for interfacing logic circuits of the general current mode logic class (circuits switching current between two outputs) and other logic circuits most often having but a single current path output.
Various kinds of logic circuit families each have their own advantages and disadvantages. Quite often, the optimum solution for a logic system design, particularly in monolithic integrated circuits, is to use logic gate circuits from more than one kind of logic family in accomplishing the system design. This poses some difficulties because the logic gate circuits in each different logic family will be operated at different logic state voltage levels and these levels, in each instance, will have a different voltage value separating them. This requires some sort of transition circuit to permit the logic signals, obtained from logic gate circuits in one kind of logic family, to be applied to logic gate circuits of another kind of logic family.
However, the design of such transition circuits must not seriously compromise the performance of the entire logic system with its gates taken from various logic families. Therefore, the transition circuit must operate with approximately the same switching rapidity as do the logic gate circuits in each of the logic family types being interfaced. Thus, when two logic family types, which are intended to have logic gate circuits taken from each interfaced, are each capable of very rapid switching, the transition circuit must also be capable of very rapid switching. Yet, at the same time, this transition circuit must provide the transition between the various voltage levels normally employed in each of the two logic family types.
Finally, when the logic family types are to be mixed in a single monolithic integrated circuit chip, the transition circuit must be compatible with the fabrication process for the forming of the other kinds of logic gate circuits present in the chip. Also, the space taken by the transition circuit being formed in the monolithic integrated circuit chip should be as small as possible to aid in achieving a high packing density of logic functions in the chip.
Some examples of prior art logic gate circuits from different current mode logic families are shown in FIGS. 1A and 1B. The circuit shown in FIG. 1A is well known as a logic gate capable of performing both the OR and the NOR logic functions. This circuit is a logic gate in what has come to be called the Current Mode Logic (CML) logic family.
Another logic gate circuit of a different current mode logic family type is shown in FIG. 1B. This logic gate can also provide both the OR and NOR logic functions. This is a logic gate in what has come to be known as the Emitter Coupled Logic (ECL) logic family.
Logic gates where the signal output is available normally from only a single current path output circuit are quite common. For instance, there are the well known Resister Transistor Logic (RTL), and Direct Coupled Transistor Logic (DCTL) logic families. These logic families are often implemented using bipolar transistors, but also, the DCTL logic family is commonly implemented using MOS transistors. More recent single output circuit logic families are shown in FIGS. 2A and 2B.
FIG. 2A shows a logic circuit from what has come to be known as the Direct Coupled Transistor-Transistor Logic (DCT.sup.2 L) logic family. This logic gate performs the NOR logic function. The logic gate in FIG. 2B is the typical logic gate circuit in the Diode Follower Logic (DFL) logic family. This gate also provides the NOR logic function. A resistor is sometimes added as shown by the dotted-in resistor appearing in this latter figure.
The logic gate circuits shown in FIGS. 1 and 2 each typically operate best at voltage levels which differ from that of every other. As a result, the pair of logic state voltage levels, characteristic of each logic family, differ between logic family types as do the voltage differences, i.e. voltage separations, separating these characteristic voltage levels. (These voltage separations are equivalent to the logic state voltage swings.) Thus, the mixing of the logic family types of FIGS. 1 and 2 in the same logic system design, particularly on the same monolithic integrated circuit chip, presents compatability difficulties. These difficulties must be resolved by a rapidly switching transition circuit.